1. Field of the Invention
The present invention relates to an axle driving apparatus including a hydrostatic transmission (HST), axles and a transmitting mechanism transmitting power to the axles, all of which are integrally provided in a housing of the axle driving apparatus. A power take off (PTO) transmission system for driving four wheels of a vehicle is compactly included in the housing.
The axle driving apparatus of the present invention also relates to the placement of a filter for filtering supply oil for the HST, and to the placement of a reserve tank regulating the volume of oil which varies with an increase of temperature of the HST.
Furthermore, the axle driving apparatus of the present invention relates to a compact arrangement of a differential locking system, which prevents bending or tilting of a differential input gear under heavy loads.
2. Related Art
A conventional axle driving apparatus includes axles and a transmitting mechanism interconnecting an HST to the axles housed in a housing. For example, U.S. Pat. No. 5,456,068, discloses an axle driving apparatus wherein an input shaft, namely a pump shaft of an HST, is disposed vertically and an output shaft, namely a motor shaft of the HST, is disposed horizontally and parallel to the axles.
Conventionally, a PTO unit is attached to such an axle driving apparatus described above. U.S. Pat. Nos. 5,367,877 and 5,515,677, for example, disclose a PTO unit separate from an axle driving apparatus and attached to a rear surface of the axle driving apparatus, projecting from the rear of the axle driving apparatus.
In cases where the input shaft of the HST is disposed vertically, as in the above-mentioned patents, the housing is vertically shorter, however, it is longitudinally and laterally longer. The input shaft necessarily projects upward, thereby requiring a predetermined space for a pulley or a sprocket above the housing. In cases where the axle driving apparatus has a vertically disposed input shaft and a PTO shaft, directional control of the axle driving apparatus requires a gear train of bevel gears or similar means because the PTO shaft is necessarily disposed horizontally. Thus, a housing including such a gear train is large. Furthermore, a casing supporting the PTO shaft projects rearwardly thereby also requiring a large space.
In an operating oil circulation system of an HST housed in a conventional axle driving apparatus, an oil filter for filtering operating oil of the HST is located in a lower portion in the housing. A reserve tank is conventionally located in an upper portion of the housing or adjacent the axle driving apparatus via piping, whereby the reserve tank is distant from the oil filter.
Accordingly, the mounting structure of the reserve tank is separate from that of the oil filter. Also, when the oil filter needs to be removed due to clogging or periodic maintenance, it can be removed only after all the operating oil is drained from the housing. Thus, removing the oil filter is a time consuming, costly and complicated process.
Furthermore, conventional differential gearing of the transmitting mechanism interconnecting the HST with the axles housed in the housing is occasionally provided with a differential locking system to restrict rotation of the differential and enable the vehicle to easily travel on a soft field.
To lock the conventional differential gearing, a differential casing is locked with a differential input gear, a differential side gear and a differential pinion via pins or other suitable means. A conventional driving apparatus housing is large when a differential casing is also disposed in the housing along with the HST, the axles and the transmitting mechanism. However, if the differential locking system is not provided with a differential casing, the differential input gear lacks adequate strength and support in cases of heavy traction. In this case, the differential input gear generates eccentric forces due to partial engagement of teeth thereof thereby reducing durability of the differential gearing.